This invention relates to a swivel and particularly a swivel used in water and oil well drilling. The main purpose of a swivel is the support for the enormous weight of a drill pipe as it is being turned, which requires the swivel to have bearings of sufficient capacity to support the pipe load. An equally important purpose of the swivel is to convey the drilling fluid from a stationary supply to the rotating pipe. Swivels are presently in use to accomplish these purposes; however, normal drilling is carried out at relatively low rotational speeds, and when the speed exceeds 300-400 rpm, several problems are encountered. The first problem is the maximum bearing speed is exceeded, and the second is the washpipe seal velocity is exceeded. This invention addresses both of these problems.
Thrust bearing speed of a swivel primarily is limited by the sliding friction of the rollers against the shoulder of the race, which is required to resist the effect of centrifugal force produced by the rollers themselves. As the rotational speed increases, the centrifugal force increases, and the frictional force increases, thereby producing ever increasing heat. The present invention overcomes this problem by the unique arrangement of two thrust bearings rather than one as is customary in the prior art. The sleeve which interconnects the swivel to the drill pipe has a drive gear formed thereon.
A support flange carries the load from the sleeve down to the upper bearing. This bearing in turn transmits the load through its races and roller assembly, through a middle race gear, to the lower bearing assembly, and finally into the swivel housing.
Before continuing, consider first a prior art swivel having only one bearing, with a sleeve speed of 1200 rpm. The rollers will have a velocity of one-half that, or 600 rpm, relative to the axis of the sleeve. The roller speed about their own axis is much higher, but the rpm that affects the centrifugal force against the shoulder radius is the one relative to the axis of the sleeve.
Consider what would happen if the stationary race of the above prior art bearing could be turned at the same speed but in opposite direction of the race powered by the sleeve. The rollers would turn relative to their own axis but would remain stationary relative to the axis of the sleeve or the bearing assembly. On the other hand, if it were turned at half speed and in the opposite direction, it would have the effect of changing the roller rpm relative to the axis of the bearing from 600 rpm to 300 rpm, and the centrifugal effect would be equivalent to that of a bearing assembly running at half speed. In this example, the fatigue life of the bearing would be reduced because the relative velocity and the number of load cycles has increased from 1200 rpm to 1800 rpm, therefore the bearing size must be increased to compensate for this change.
In the present invention, the second thrust bearing is required to allow for the rotation of both races in the top bearing. Its speed would be the same as the lower race of the upper bearing, which in the case above would be 600rpm, but the life would be much greater as its life is based also on 600 rpm. If desired, this lower bearing could be a smaller capacity bearing and still be an equivalent bearing.
In this invention, a novel drive arrangement is shown to provide the turning of the inner races which would be designed to run the inner races in the opposite direction of the sleeve and at half speed, for example.
The washpipe seal problem is based upon flow requirements and peripheral velocity of the washpipe surface riding against the washpipe seal. Since the pressure of the drilling mud to the improved swivel will be approximately the same as the prior art, the velocity of the rotating washpipe against the stationary seal must be changed. One way to accomplish this is to reduce the diameter of the washpipe. However, in most cases, this would not be possible as the pressure drop through this diameter determines the amount of fluid that can be pumped to the bit. In the prior art configuration, one end of the washpipe is held stationary and the packing attached to the sleeve rotates on the other. In this invention, packing is placed on both ends of the washpipe, and the washpipe is rotated at half the speed, but in the same direction as the sleeve, thus reducing the effective peripheral sleeve velocity to one half that of the prior art washpipe arrangement.
Accordingly, the present invention provides method and apparatus by which a prior art drill string can be rotated at about twice the rotational speed presently allowed by a swivel.